Nervous-system or “C N S” fatigue has become a modern-day buzzword right up there with overtraining. Don’t train too hard or you’ll burn out the nervous system. I’m at least partly to blame for spreading awareness of this phenomenon, and don’t mistake me, it’s a real thing, but the concept has mutated beyond recognition.
Sports science, on the other hand, has a clearer definition. Fatigue is just a fancy way of saying that you’re tired and not operating at peak capacity. The question is, what’s getting tired? Better, is anything tired at all?
Being a body-oriented profession, we naturally focus on the heart and lungs, in endurance athletes, or muscles in the case of more strength-dependent activities. We easily relate to tired muscles, legs made of jelly after a long run, or when you can’t raise your arms after a hard shoulder workout. The muscles themselves have been worked into paste and can no longer sustain the activity. Or, in the case of endurance athletes, you’ve reached your VO2 max and can no longer keep up the pace because your lungs are burning.
When the tiredness happens in the working tissues, we call this peripheral fatigue.
But there’s more. Sports scientists noticed something fishy while playing around with electro-stim devices, which apply an electric current to a muscle and cause it to contract involuntarily. When you do this to a fatigued muscle, the contraction isn’t quite as hard as you’d expect in a fresh counterpart. This makes sense, since the muscle itself is tired and forcing it to contract won’t change that.
Under some circumstances, though, you can apply current to a tired person and the muscle contracts just fine, like it’s not tired at all. Yet when asked to contract the muscle voluntarily, the subjects can’t do it. Researchers labeled this central fatigue, since there’s no obvious cause of tiredness in the muscles. The loss of performance is due to central causes ― that is, happening in the brain or spinal cord, better known as the central nervous system (CNS).
South African sports scientist Timothy Noakes suggested an explanation for this phenomenon, focusing on the brain and its damage-control features. According to Noakes, we experience fatigue as a specific sensation that alters our perception of effort as our bodies do work and grow tired during physical activity, and we can measure this conscious perception of difficulty with the rating of perceived exertion (R P E), a value that rates how hard you’re going compared to your theoretical best-effort. We’ll see a lot more about this later.
Noakes’s central governor hypothesis says that the feeling of difficulty, measured by the R P E, gradually increases during a workout, and in response our neural output ― central drive to the working muscles ― drops off. We’re often physically capable of doing much more work, at a higher effort, than we typically do, but from a survival standpoint, voluntarily working to a point of catastrophic failure isn’t the best idea.33
Noakes suggests that the brain pulls back on the throttle as a protective measure. The
sensation of “tiredness” is our psychological experience of this protective mechanism.
Our limits, both in endurance and in maximum intensity, are in part physical and in part psychological (although in reality there is no distinction between the two, as I’ve suggested).
The governor isn’t a particular cluster of neurons or any part of the brain that we can point to and say “fatigue happens here”. Like many brain functions, the governor is intended as a shorthand for the behavior of many (many) networks spread throughout the brain acting together. Whatever the governor really is, if Noakes is right it acts like one of Antonio Damasio’s somatic markers, taking feedback from the body ― from muscle tissue and the cardiovascular system in this case ― and integrating it into a bodily sensation.
Fatigue can be triggered by a stunning range of signals, as everything from ammonia and oxygen content in your blood to the availability of neurochemicals in your brain and the feedback from receptors in your muscles and joints works into the calculation of just how tired you are.
This is the kind of fatigue that you can overcome by reaching down and digging in your heels. You can make that last quarter mile; you can grind out that 10kg squat P R if you grit your teeth and keep it moving. Since nothing is actually “tired” in the way we tend to think, you can grunt your way through it with an executive override. It won’t be pleasant, however, because you’re working against the survival instincts built into your brain.
Intriguingly, mental fatigue by itself can trigger the fatigue effect without any need for you to actually do anything. A 2009 paper from Samuele Marcora’s team at Bangor University in Wales tested the effects of mental fatigue on high-intensity cycling. Subjects given a complicated mental task before the cycling test couldn’t keep up the pace compared to the control group.34
Marcora’s team suggests that mentally-demanding tasks fatigue the anterior cingulate cortex (ACC), another part of the brain important for its role as a junction between your body-sense and your conscious perception of effort. During exercise, the autonomic nerves fire on all cylinders to keep heart rate and blood pressure and everything else working in “exercise mode”. The ACC plugs that information in to our conscious minds and we experience it as “hard work”.
When you go exercise after spending half the day studying differential equations, you’ve worn out your ACC and the exercise feels much harder than it should. Marcora argues that the perceived difficulty itself may cause you to cut off a workout before any genuine physical fatigue sets in.35
There’s no “handbrake” in the brain as in Noakes suggests. Instead, as we exercise and gradually tire out, neural output from the brain has to increase to keep up the pace.
That increased output translates into feelings of difficulty. Fatigue happens when the feeling out-does your motivation to keep going and to ignore the pain.36
We don’t have to worry over the nuances of the scientific back-and-forth. The
competing models differ in detail but agree on the key point: the brain is receptive to physical signs of fatigue as well as being the site of mental fatigue. The two fatigue processes seem to share many of the same circuits, and both involve changes in brain activity that we experience, subjectively, as feelings of difficulty and a loss of performance. Likewise, those same fatigue-induced changes lead directly to “coping behavior”.
Or, more simply: As exercise feels harder, whether from mental or physical tiredness, you’re more likely to stop doing it and it’s more likely to make you feel bad afterwards.
Calling fatigue an altered brain state or an illusion of the senses isn’t meant to discount the feeling. The sensation itself may be “just a feeling”, but the change in brain state, and the reduction of neural output, most assuredly is not. Fatigue markers can impair performance just as sure as any injury.37
Confirming this, Romain Meeusen of Belgium’s Vrije Universiteit has shown that the feeling of fatigue happens as a consequence of altered neurological activity, specifically the behavior of two important neurotransmitters.38
Back in 1987, Eric Newsholme proposed that transmission of serotonin in the brain increases during exercise, leading to feelings of lethargy and contentment as well as a perception of fatigue. Newsholme called this the serotonin hypothesis of fatigue.
Meeusen’s research found that elevated serotonin is only half of the central fatigue equation. Dopamine, another neurotransmitter involved in motivation and motor control, also increases during intense exercise. But Meeusen found that, at the point of exhaustion, dopamine levels drop off sharply. We experience central fatigue after that dopamine crash, while serotonin is still high, and it fs this ratio between dopamine and serotonin that matters in our perception of tiredness.39
Meeusen warns that there is no one pathway that completely governs fatigue.
Serotonin and dopamine, while likely an important piece of the puzzle, are only two players in a complex game of arousal and inhibition. He points to the brain’s store of glycogen fuel, which is a paltry 1.5 grams when maxed out, as another factor worth consideration. The brain fs energy usage is so high that even small changes in glycogen make a big difference in our perceived energy levels. Intense focus and concentration depletes brain glycogen, perhaps explaining why studying all night, or spending 10 hours behind the wheel, wears you out.